The present invention relates to improving the low load operation of fuel burners for use in pulverized coal-fired furnaces and, more particularly, to improving low load operation of fuel-air admission assemblies for directing a pulverized fuel-air mixture into the furnace by what is known as the tangential method of firing.
In view of today's fluctuating electricity demand, typified by peak demand occurring during weekday daytime hours and minimum demand occurring at night and on the weekends, electric utilities have chosen to cycle many of their conventional coal-fired steam generator boilers by operating them at full load during peak demand hours and reducing them to low loads during periods of minimum demand.
As a consequence of this mode of operation, the electric utilities have used large quantities of natural gas or oil to furnish additional ignition energy during low load operation because the current generation from coal-fired steam generator furnaces require stabilization of the coal flames when operating at low loads. The required amount of auxiliary fuel fired for stabilization purposes is significant and, for example, to maintain a 500 megawatt coal-fired steam generator at 10 to 15 percent load during minimum demand periods could require the use of 11,000 gallons of oil per day.
One common method of firing coal in conventional coal-fired steam generator boilers is known as tangential firing. In this method, pulverized coal is introduced to the furnace in a primary air stream through burners, termed fuel-air admission assemblies, located in the corners of the furnace. The fuel-air streams discharged from these assemblies are aimed tangentially to an imaginary circle in the middle of the furnace. This creates a fireball which serves as a continuous source of ignition for the incoming coal. More specifically, a flame is established at one corner which in turn supplies the required ignition energy to stabilize the flame emanating from the corner downstream of and laterally adjacent to it. When load is reduced, the flames emanating from each corner become shorter and, as a consequence, a reduction in the amount of ignition energy available to the downstream corner occurs. As a result, auxiliary fuel such as oil or natural gas must be introduced in each corner adjacent to the pulverized coal-air stream to provide additional ignition energy thereby insuring that a flameout and resultant unit trip will not occur.
Another problem associated with operating a coal-fired burner at low loads results in the fact that the pulverizing mills typically operate with a relatively constant air flow over all load ranges. When furnace load is reduced, the amount of coal pulverized in the mills decrease proportionally while the amount of primary air used to convey the pulverized coal from the mills through the admission assemblies into the furnace remains fairly constant, thereby causing the fuel-air ratio to decrease. When the load on the furnace is reduced to the low levels desired during minimum demand periods, the fuel-air ratio has decreased to the point where the pulverized coal-primary air mixture has become too fuel lean for ignition to stabilize without significant supplemental ignition energy being made available.
One way in which the need for auxiliary fuel firing during low load operation on coal-fired boilers can be reduced is disclosed and claimed in a related application Ser. No. 29,605 of even date of McCartney entitled, "Low Load Coal Bucket". The McCartney invention, which addresses both of the aforementioned problems, provides an improved fuel-air admission assembly incorporating a split coal bucket which permits a pulverized coal-fired furnace to be operated at low loads without use of auxiliary fuel to provide stabilization.
In accordance with the McCartney invention, the split coal bucket comprises an upper and a lower coal nozzle pivotally mounted to the coal delivery pipe, the upper and lower coal nozzles being independently tiltable. When the furnace is operating at low loads such as during the minimum demand periods, the primary air and pulverized coal stream discharging from the coal delivery pipe to split into an upper and a lower coal-air stream and independently directed into the furnace by tilting the upper coal nozzle upward and the lower coal nozzle downward. In doing so, an ignition stabilizing pocket is established in the locally low pressure zone created between the spread apart coal-air streams. Hot combustion products are drawn, i.e., recirculated into this low pressure zone thus providing enough additional ignition energy to the incoming fuel to stabilize the flame.
Ignition stability is further improved by the fact that as the upper and lower coal-air streams split, the coal in the upper coal-air stream tends to concentrate along the lower surface of the upper coal-air stream as a result of the density differential between the coal and air and the centrifugal forces generated as the upper coal-air stream is turned upward. Similarly, the coal in the lower coal-air stream tends to concentrate along the upper surface of the lower coal-air stream as the lower coal-air stream turns downward when passing through the lower coal nozzle. Since the lower surface of the upper coal-air stream and the upper surface of the lower coal-air stream border upon the ignition stabilizing zone established therebetween, the concentrated coal will be drawn into the ignition stabilizing zone thereby increasing the local fuel-air ratio above the fuel-air ratio present in the primary air and pulverized coal mixture leaving the pulverizer at low loads which, as explained above, is too fuel lean for ignition to stabilize without significant supplemental ignition energy being made available. By increasing the local fuel-air ratio in the low pressure ignition zone established between the spread apart coal-air streams, the need for supplemental energy for stabilizing ignition is reduced.
The present invention is an improvement upon the above-described McCartney invention and is directed at further increasing the local fuel-air ratio in the low pressure ignition zone established between the spread apart coal-air streams, thereby further reducing the need for supplemental energy for stabilizing ignition of the coal flame during low load operation.